1. Technical Field of the Invention
The present invention relates to retractable antennas, and more particularly, to an apparatus for connecting an impedance matching network with a retractable antenna.
2. Description of Related Art
The performance of an antenna is determined by its impedance which is dependent upon its wavelength. A retractable antenna inherently performs differently in the extended and retracted positions since the effective wavelength of the antenna is greater in the extended position then in the retracted position. Presently existing retractable antennas normally consist of a quarter wavelength helical coil connected with a quarter wavelength rod. The amplifiers connected to antennas normally are matched to approximately a 50 .OMEGA. output impedance. When the antenna is retracted, the quarter wavelength rod is shorted to ground while the quarter wavelength helical coil is directly connected to the amplifier output. The load impedance provided by the quarter wavelength helical coil is approximately equal to the 50 .OMEGA. load impedance required by the amplifier. Thus, the impedances match and maximum signal transfer is achieved. However, when the antenna is extended, the quarter wavelength helical coil and quarter wavelength rod present a high load impedance for connection to the amplifier output. This creates unequal impedance matches between the load impedance of the antenna and the load impedance required by the RF amplifier.
To produce similar antenna performance in both the extended and retracted positions, an impedance matching network must be switched into place when the antenna is in the extended position to match to the impedance load of the antenna. Present solutions to this problem have incorporated an electromechanical switch connector to connect high and low impedance matching circuits between the antenna and the amplifier. The quarter wavelength rod portion of an antenna includes upper and lower contact points. In the extended antenna position, the lower contact on the quarter wavelength rod contacts the connector for a high impedance matching circuit connecting the high impedance circuit between the antenna and the amplifier. In the retracted position, the upper antenna contact connects with a low impedance matching circuit, while the low contact connects with a ground connector. This effectively isolates the quarter wavelength rod from the amplifier and provides an equivalent low impedance connection from the helical coil to the output of the amplifier.
However, this solution suffers from several drawbacks. The connectors of this type of network are sensitive to corrosion, fatigue, and tolerance buildup. Thus, they have a high degree of likelihood of mechanical failure. Furthermore, testing of a radio telephone during manufacture is difficult with this type of network, since the impedance matching network is only activated by the insertion of an antenna element into the radio telephone. Thus, no convenient 50 .OMEGA. RF feedpoint at the radio telephone is available for testing. It is highly desirable to include a 50 .OMEGA. feed point at the antenna port that does not include any matching networks for the antenna. Thus, an antenna impedance matching network that requires no switch contacts and enables connection of test equipment directly to a 50 .OMEGA. output feed point during manufacture would be highly desirable.